The present invention relates to an MIG (Metal Inert Gas) welding process for welding nickel alloys efficiently and to special gas mixtures intended for such a process.
In MIG welding, the first function of the welding gas is to jointly shield the molten metal which transfers in the arc and comes from the melting of the end of the meltable electrode wire, and the weld puddle consisting of deposited metal and base metal. Argon is an inert gas often used for this purpose.
However, for the MIG welding of steel, it is known that argon alone is insufficient to optimize the process. This is because, beneath argon alone, the arc and the metal transfer in the arc are unstable.
To remedy this problem, it is conventional to add an oxidizing gas to the argon, this having the effect of stabilizing the root of the arc, that is to say the cathode spot, since the slight surface oxidation resulting from the presence of oxygen then makes the materials to be welded more emissive. Thus, more stable and spatter-free welding and a more uniform weld bead are obtained.
As oxidizing gas or stabilizing element, oxygen or carbon dioxide may be used, the nature and the content of the stabilizing element varying according to the grade or the composition of the materials to be welded.
In the case of carbon steels for example, the carbon dioxide (CO2) content varies from a few % to 100% by volume, more usually from 5 to 60 vol %.
In contrast, in the case of stainless steels, for which it is necessary to limit the surface oxidation of the material and the carbon content of the metal deposited, the carbon dioxide content is generally between 1 and 3 vol %, depending on the transfer mode adopted. However, if the oxidizing element is oxygen, since its oxidizing power is greater than that of carbon dioxide, the amount added may be less than these values.
However, at the present time a problem arises in the case of the MIG welding of nickel and nickel alloys.
Nickel and its alloys have two main characteristics which are the basis of why they are used in the chemical, petrochemical, nuclear, aeronautical and space fields, namely:
good corrosion resistance in very varied media. As in the case of stainless steels, it is the addition of chromium which gives the alloy its corrosion resistance, it being possible for this characteristic to be enhanced by the presence of chromium and molybdenum; and
good high-temperature resistance, including in an oxidizing medium.
Ignoring the metallurgy of the welding and focusing simply on the operating aspect, that is to say the melting of the wire and the shielding of the molten metal, when welding nickel and its alloys there is the particular feature, compared with stainless steels, that the molten metal has a higher viscosity due to the effects of nickel and molybdenum on the surface tensions of the puddle, and it is more difficult to protect the molten metal remaining at high temperature from oxidation.
This is manifested in general by weld beads that may be domed and irregular, these being characteristic of a metal exhibiting poor wetting and an unstable arc, resulting in an energy density which is too low and poorly distributed.
If the gaseous shielding means employed are insufficient, the weld bead may also be highly oxidized to the point of having blackish adherent coloration difficult to remove.
Consequently, it is an object of the present invention to provide a process for the MIG welding of nickel and its alloys which makes it possible to produce a welded joint with correct and continuous surface wetting over the entire length with substantially reduced surface oxidation.
In other words, it is an object of the invention to provide a process for the MIG welding of nickel and its alloys using a shielding gas mixture ensuring greater arc stability, higher energy density and better wetting of the weld bead than usually obtained by this process.
This is because, on the one hand, a more stable and more concentrated arc makes it possible to increase the penetration (depth) and therefore, for a given thickness, to increase the welding speed and, on the other hand, by very significantly reducing the surface oxidation of the weld beads, which remains acceptable, it is possible to dispense with the usually necessary operations of grinding or brushing in multipass welding or in weld bead finishing.
The invention therefore relates to a process for the MIG welding of nickel and nickel alloys, with the use of a gas shield for at least part of the welding zone, in which process the gas shield is a gas mixture containing (by volume) from 0.05% to 0.5% CO2, the rest being argon.
Depending on the case, the process of the invention may include one or more of the following characteristics:
the gas mixture furthermore contains, by volume, from 15 to 50% helium, preferably from 17% to 30% helium and preferably less than 25% helium;
the gas mixture furthermore contains from 0.1% to 10% hydrogen by volume and preferably from 1% to 7% hydrogen by volume;
the gas mixture contains from 0.1% to 0.5% CO2, preferably from 0.1% to 0.4% CO2 and even more preferably from 0.1% to 0.3% CO2;
the gas mixture consists, by volume, of 0.1 to 0.5% CO2, of 16 to 19% helium and of argon for the rest, preferably around 0.3% CO2, around 18% helium and argon for the rest;
the gas mixture consists, by volume, of 0.1 to 0.3% CO2 and of argon for the rest;
the gas mixture consists, by volume, of 0.1 to 0.3% CO2, of 15 to 50% helium and of argon for the rest;
the gas mixture consists, by volume, of 0.1 to 0.3% CO2, of 1 to 5% hydrogen and of argon for the rest, preferably of 0.1 to 0.3% CO2, of 3 to 5% hydrogen and of argon for the rest;
a solid or flux-cored meltable wire is used, the grade of which is chosen depending on the alloy family or families in question, that is to say depending on the nickel alloy to be welded;
the process is carried out in short-arc, pulsed or axial spray transfer mode depending on the nature of the wire used (solid or flux-cored).
The invention also relates to a gas mixture consisting, by volume, of 0.1 to 0.3% CO2, of 1 to 5% hydrogen and of argon for the rest, preferably of 0.1 to 0.3% CO2, of 3 to 5% hydrogen and of argon for the rest.
In addition, the invention also relates to a gas mixture consisting, by volume, of 0.1 to 0.3% CO2, of 15 to 50% helium and of argon for the rest, to a gas mixture consisting, by volume, of 0.1 to 0.3% CO2 and of argon for the rest and to a gas mixture containing (by volume) from 0.05% to 5% CO2, and the rest being argon, preferably from 0.1% to 4% CO2, the rest being argon, and even more preferably from 0.1% to 1.5% CO2, and the rest being argon.
Furthermore, the invention also relates to the use of one of the above gas mixtures to produce a coating made of nickel or a nickel alloy on at least one metal workpiece made of carbon steel, stainless steel, nickel or a nickel alloy, preferably a workpiece made of nickel or a nickel alloy.